1. Field of the Invention
The present invention relates to bandpass filter units and communication apparatuses, and more particularly, to a bandpass filter unit that is configured to suppress reflected waves at input and output terminals.
2. Description of the Related Art
Bandpass filters have been used in transmission circuits or signal processing circuits in high frequency bands, such as microwave and millimetric wave bands. This type of bandpass filter includes, for example, surface acoustic wave (SAW) filters, dielectric filters, waveguide filters, microstrip line filters, and filters including lumped constant components having reactance (capacitors, chip inductors, air-core coils, etc.).
FIG. 12 is a schematic plan view illustrating a resonator SAW filter unit 500 as an example of known bandpass filters. In the resonator SAW filter unit 500, a resonator SAW filter 502 is provided on a piezoelectric monocrystal substrate 501. The resonator SAW filter 502 includes interdigital transducers (IDTs) 514, 515, and 516 arranged in a direction in which a SAW propagates, and reflectors 513 and 517 disposed in the SAW propagating direction such that they sandwich the IDTs 514, 515, and 516 therebetween.
An electrode pad 509 connected to a ground potential and an electrode pad 512, which defines an output terminal, are connected to the central IDT 515 via connecting conductors 504 and 507, respectively. An electrode pad 511, which defines an input terminal, is connected to one comb-like electrode of the IDT 514 and one comb-like electrode of the IDT 516 via connecting conductors 503 and 505, respectively. An electrode pad 510 connected to a ground potential is connected to the other comb-like electrodes of the IDTs 514 and 516 via connecting conductors 506 and 508, respectively.
As the resonator SAW filter unit 500, a bandpass filter having a characteristic impedance of 50Ω and a pass band of 1805 MHz to 1885 MHz is provided. Examples of the characteristics of this bandpass filter are shown in FIGS. 13 through 16. FIG. 13 illustrates a transmission characteristic; FIG. 14 illustrates the enlarged essential portion of the transmission characteristic shown in FIG. 13; FIG. 15 is a Smith chart illustrating an impedance characteristic of the input terminal of the resonator SAW filter unit 500 in the pass-band frequencies; and FIG. 16 is a Smith chart illustrating an impedance characteristic of the output terminal in the pass-band frequencies.
The principle of the operation of the resonator SAW filter unit 500 is described in, for example, “SAW Device Technique Handbook” (edited by the SAW Device Technique 150th Committee of the Japan Society for the Promotion of Science, and published by Ohm-sha, Ltd.).
In bandpass filters used in microwave or millimetric bands, perfect impedance matching is preferably provided at input and output terminals. That is, it is desirable that signals in the pass band do not reflect at input and output terminals. This is because loss may be caused in reflected signals in the pass band, and also, reflected waves may produce an adverse influence on electric circuits connected to the bandpass filters.
The above-described problem is more specifically explained below in the context of a bandpass filter disposed between an antenna and an amplifier in a receiver of a cellular telephone.
If impedance matching is not provided for pass-band signals at the input terminal of a bandpass filter, the portion of a pass-band signal received by the antenna is reflected at the input terminal of the bandpass filter, thereby causing loss in the received signal. In this case, to compensate for the loss in the received signal, the gain of the amplifier must be increased for ensuring necessary signal intensity, resulting in an increase in the power consumption of the cellular telephone.
Also, loss in a portion of the received signal decreases the signal-to-noise (S/N) ratio. Even if the signal level is increased by the amplifier at the subsequent stage, the S/N ratio is not recovered, thereby impairing the reception performance of the cellular telephone.
The portion of the received signal reflected at the input terminal of the bandpass filter is also reflected at the antenna terminal, and is returned to the bandpass filter. Accordingly, due to this multiple reflection, a received signal having a phase delay is superposed on the normal received signal in the bandpass filter. Thus, due to this multiple reflection, the level of the received signal is also decreased, and the reception performance of the cellular telephone is impaired.
As described above, the reflection of signals at the input terminal of the bandpass filter causes various adverse influences. It is thus demanded that the impedance matching be provided in the pass band so as to suppress the reflection of signals to a minimal level at the input terminal of the bandpass filter.
If the impedance matching is not provided in the pass band at the output terminal of the bandpass filter, multiple reflection occurs in the pass-band signals between the output terminal of the bandpass filter and the input terminal of the amplifier. Accordingly, the operation of the amplifier, which is designed to provide the gain for the pass-band signals, becomes unstable, and in the worst case, abnormal oscillation occurs. If the level of the impedance mismatching at the output terminal of the bandpass filter is not considerably high, abnormal oscillation does not occur. In this case, however, the multiple reflections occurring between the bandpass filter and the amplifier due to the impedance mismatching inhibit, more or less, the normal operation of the bandpass filter. It is thus desirable that the impedance matching be also provided at the output terminal of the bandpass filter so as to suppress the reflection of signals.
Generally, not only in this type of bandpass filter, but also in bandpass filters used in a microwave or millimetric wave band, it is desirable that the perfect impedance matching be provided at input and output terminals in the entire frequency range of the pass band so as to suppress reflection of pass-band signals at the input and output terminals.
In reality, however, it is practically impossible to provide perfect impedance matching for the input and output terminals in the entire frequency range of the pass band because the input/output impedances of the bandpass filters have frequency characteristic. It is thus important that almost perfect impedance matching be provided in the entire frequency range of the pass band.
The impedance characteristics at the input and output terminals of the known resonator SAW filter unit 500 indicate, as shown in FIGS. 15 and 16, that the perfect impedance matching is not provided although the impedance is positioned close to the perfect matching point, which is the center of the Smith chart. That is, the impedance characteristic moves around the perfect matching point while exhibiting frequency characteristic. Accordingly, the reflection of the signals occurs at the input and output terminals in accordance with the distance of the input/output impedances to the perfect matching point.